While the judgement of a science fair project is subjective, elements such as objectives, skills demonstration, data and interpretation generally garner high marks. The display of the project should be a reflection of the student's personality, which makes it stand out from the crowd.
The center of any winning science fair project is a clearly stated objective. The student needs to clearly describe the problem and the method for researching the topic.
The purpose of a science fair project is to encourage students to practice their skills related to science. As such, judges are often looking for demonstrations of these skills. They want to see that the students did the experiments themselves and have a good command of the technical parts of the project.
Similarly, students should have a clear understanding of the data. They should use a journal or some other organized method for collecting data from the experiment. Likewise, it's recommended that students repeat experiments to check the reliability of their data.
The interpretation of the data is also very important. Whenever possible, students should use tables and graphs to display their findings.
Finally, students need to create a winning display. Typically this takes the form of a tri-fold panel. Discovery Education suggests using the center of the tri-fold to chronicle the experiment or investigation precisely. Likewise, students should complete all writing and drawing in pencil before finalizing the display with pen.